Browsing by Author "Brooks, Philip R."
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Item Collisional ionization studies of rubidium atoms and small halogenated molecules near reaction thresholds(2006) Jia, Beike; Brooks, Philip R.An atomic supersonic beam of Rb (seeded in H 2 ) and a gas beam (organic molecules seeded in He or Ar) are crossed to investigate electron transfer reactions close to their energy thresholds, and to measure the electron affinities (EA) of various product negative ions. The ions are collected and identified by coincidence TOF mass spectrometry. The signal from the Rb seeded beam is orders of magnitude higher than that from a Rb charge exchange sources. Most of the energy comes from the Rb beam. Therefore the speed distribution of this beam is measured by time of flight and the speed of the gas beam estimated from theory. Then the energy distribution for the reaction is calculated. The coincidence signals of negative ions are analyzed from fundamental kinetic theory with the reaction threshold an unknown parameter in the equation. A computer program fits the experimental data and finds the optimal value of the threshold. The EA's of all negative ions are then calculated by conservation of energy. C 2 F 5 Cl, C 2 F 5 I, C 6 F 6 , C 2 Cl 4 , C 2 HCl 3 , Cl 2 C=CH 2 , trans-C 2 H 2 Cl 2 and many other molecules have been explored. The experiments produce various parent negative ions and fragments. The initial analysis suggests that C 2 F 5 has a surprisingly higher EA than what has been reported in the literature, but we believe this high value is due to the small fraction of Rb dimer. The parent ion of C 2 F 5 I, which has not been reported previously, is made in this experiment and its EA is determined. For C 6 F 6 the parent ion signal and a long tail electron signal are observed, indicating that the electrons are from autodetachment of the parent ions. The smaller EA compared to other work also supports this argument. The chlorine substituted ethene molecules produce Cl - and parent ions. Analysis shows negative EA's for all the parent ions which suggests they are probably formed in some excited states or the speed of the molecules is over estimated due to the slip between the seed and carrier gas (He).Item Experimental Study of Potassium and Strontium Rydberg Atoms - Chaotic Ionization, Quantum Optical Phenomena and Multiphoton Excitation(2014-04-24) Ye, Shuzhen; Dunning, F. Barry; Killian, Thomas C.; Brooks, Philip R.Very-high-n (n~300) Rydberg atoms serve as a powerful tool to study chaos and quantum optical phenomena. Measurements using a series of alternating impulsive kicks applied to potassium Rydberg atoms reveal that a phase space geometric structure called the turnstile governs the ionization process. Studies of the excitation spectra for potassium Rydberg atoms in a strong sinusoidal electric drive field in the radio frequency (100-300 MHz) regime, display quantum optical phenomena including electromagnetically induced transparency and Aulter-Townes splitting, and the data are well explained within the framework of Floquet theory. In order to study the strong dipole-dipole interactions between neutral atoms, new experimental techniques have been developed to create high densities of very-high-n (n~300-500) strontium Rydberg atoms using two- and three-photon excitation. The data demonstrate that high densities of strongly-polarized quasi-one-dimensional states can be produced and form the basis for further manipulation of the atomic wave functions. The strontium Rydberg states are modeled using a two-active-electron theory which produces results in good agreement with experimental observations.Item Molecular beam studies of excitation and electron transfer reactions(1997) Lewis, Lawrence Lyle; Brooks, Philip R.Two studies were performed using crossed molecular beams. The first system studied was the reaction $\rm Na\sp* + KBr \to NaBr + K\sp*,$ determining how fine structure is transmitted through a reactive collision. Each fine structure state of Na$\sp*(3\sp2$P) is separately laser excited, and the fluorescence from the two fine structure states of K$\sp*(4\sp2$P) are separately monitored. The observed K* fine-structure state distributions were not simply statistical. While the product K* fine-structure states were statistically populated for excitation to Na$\rm\sp*(P\sb{1/2}),$ they were not for excitation to Na$\rm\sp*(P\sb{3/2}).$ These distributions were interpreted in terms of nonadiabatic interaction along different regions of the KBrNa molecular potential energy surfaces. These nonadiabatic interactions were also used to help explain the differing fine-structure state populations produced in the previous NaBr + K transition state spectra. A hyperthermal seeded supersonic alkali atom source was designed and constructed for use in collisional ionization experiments. The intensity of the new source was found to be ${\approx}10\sp5$ greater than the previous charge exchange source in the energy range of interest. This source was then used to determine preliminary appearance thresholds for collisional ionization between potassium and rubidium atoms and some molecules. From the thresholds, electron affinities for SF$\sb6$ and CF$\sb3$Br and the bond dissociation energy for the CH$\sb3$Br bond could be obtained. These values were in good agreement with the literature values, although the electron affinity for CF$\sb3$Br was slightly higher (1.06 $\pm$ 0.10 eV) than the previous result in the literature (0.91 $\pm$ 0.20 eV). The effect of electronic excitation of the alkali atom on collisional ionization was also explored. The cross section for the reaction of excited state $\rm Rb\sp* + SF\sb6$ appears to be less than that for the ground state reaction. This reduction in cross section suggests that the excited state crossing can be considered to be nearly completely nonadiabatic for the experimental conditions. This result was reproduced in Landau-Zener calculations of the nonadiabatic probabilities and the ratio of the cross sections.Item Orientation effects in cross-beam ionization reactions between potassium and symmetric-top molecules(1993) Xing, Guoqiang; Brooks, Philip R.Symmetric-top molecules (CF$\sb3$Br, CF$\sb3$Cl, CF$\sb3$H and CH$\sb3$Br) in a seeded supersonic nozzle beam are orientation selected by a hexapole electric field, and collide at a right angle with fast (3-40 eV) potassium atoms. The ionization reactions at two different molecular orientations are studied: $$\eqalign{&\rm K + CX\sb3 - Y \to K\sp+ + CX\sb3 + Y\sp-\qquad (Tails\ Orientation)\cr &\rm K + Y - CX\sb3 \to K\sp+ + CX\sb3 + Y\sp-\qquad (Heads\ Orientation)\cr}$$ We observed that collision ionization reactions are influenced greatly by molecular orientations. For CF$\sb3$Br, CH$\sb3$Br and CF$\sb3$Cl, the reactivities are greater with the heads orientation than that with the tails orientation, but for CF$\sb3$H, the H end is unreactive. The steric effects are more pronounced at the low energy end near the thresholds, and almost disappear at energies above 20 eV. Most importantly, we also found that the energy thresholds of these reactions are different for heads and tails orientations, indicating that the electron affinity of a molecule should be considered as an anisotropic parameter. Some features of the experimental results are explained by the Harpoon Electron Transfer model. Further theoretical and experimental studies are required for the fully understanding of the reaction dynamics.Item Robert F. Curl, Jr.: Physical Chemist and Codiscoverer of Fullerenes(PNAS, 2022) Brooks, Philip R.; Weisman, R. Bruce; Johnson, Bruce R.Item Studies of oriented molecules using coincidence time-of-flight mass spectroscopy(1999) Wiediger, Susan D.; Brooks, Philip R.A beam of symmetric top (CX3Y) molecules is state-selected using an inhomogeneous hexapole electric field and then oriented by homogeneous electric fields. A fast neutral atomic potassium beam produced via charge-exchange crosses the molecular beam at a right angle. Continuous electric fields extract ion pair products as they are formed into dual time-of-flight (TOF) mass spectrometers, where the arrival time difference is characteristic of the mass difference of the particles. The positive product (K+) is known for these reactions, allowing identification of the negative products based on the time difference. Construction of the dual time-of-flight (TOF) mass spectrometers is described, with an explanation of how coincidence detection is used to identify the negative ion products of reactions between fast neutral K atoms and small molecules (≤7 atoms). Examination of the results of initial characterization studies including reactions with CH3Cl, CH3Br, CH3I, CF 3Br, and SF6 indicate that coincidence TOF should be a powerful tool for investigations into the dynamics of these reactions. Preliminary results suggest that orientation data will be obtainable to address the question of whether different approach vectors result in different product channels. The importance of a well-calibrated alkali source, well-defined electric fields, and a good vacuum is indicated. An appendix discusses radio-frequency (RF) spectroscopy experiments performed to explore characteristics of the rotational distribution in the molecular beam. A possible application of RF spectroscopy to "tag" single quantum states is discussed.Item The photoexcitation spectra of transition region species in reactions of potassium + sodium halides (X = chlorine, bromine, iodine)(1991) Barnes, Michael Dean; Brooks, Philip R.The photoexcitation spectra of transition region species formed in bimolecular reactions of K + NaX (X = Cl, Br, I) have been observed by measuring the intensity of sodium D line emission at 589.0 nm as a function of excitation wavelengths between 595 and 640 nm. The portion of the spectrum measured for the K + NaCl system is qualitatively similar to that previously observed by Magurie, et al (MAG86). The spectra obtained for the K + NaBr and NaI systems are significantly different than that of the K + NaCl system, and show a distinct feature centered at approximately 610 nm, and indicate that the reaction dynamics are quite different for the heavier sodium halide systems. The results of classical trajectory calculations performed using K + NaCl potential energy surfaces suggest that the structure in the K + NaBr and K + NaI spectra is not due to a mass effect; but rather from unique features of the potential energy surfaces for these reactions which are, as yet, unknown. Because of insufficient theoretical and experimental information, interpretation of these results in terms of the dynamical processes in these reactions is not yet possible. However, several different possible mechanisms of this structure are discussed which can be experimentally tested. Results of proposed future experiments should be able to provide the necessary information to understand the nature of these spectra.